Single-jet liquid meters generally comprise a body within which there is a measurement chamber of substantially cylindrical shape and equipped with a spinner rotating about a longitudinal axis and provided with a plurality of blades. The body of the meter is connected to two tubes, one for feeding liquid into the measurement chamber and another for discharging it from the chamber.
There exist meters in which the tubes are aligned with each other but are offset, i.e. their alignment axis does not intersect the longitudinal axis. This disposition of the tubes is disadvantageous given that it makes such meters difficult to install in meter premises where space is often at a premium.
There exist meters in which the tubes are likewise offset relative to the longitudinal axis of the spinner, but which are inclined symmetrically relative to said spinner instead of being in alignment with each other. Such meters require the inclination of the tubes to be modified when there is a change in length of the meter.
Finally, as shown in FIG. 1, there exist meters 2 in which the liquid feed tube 4 and the liquid discharge tube 6 are in alignment with each other and whose alignment axis YY' intersects the longitudinal axis XX' of the spinner 8 perpendicularly.
In this type of meter, provision is made to install a housing 10 inside the body, around the periphery of the spinner 8, which housing is known as an "injection box" and is generally made of plastics material. This housing has two slots 12 and 14 placed respectively facing the liquid feed tube 4 and the liquid discharge tube 6. The slots 12 and 14 are inclined symmetrically relative to the tube alignment axis respectively to bring the liquid flow to the periphery of the spinner and to discharge said flow from the periphery of the spinner.
When the spinner of such a meter is rotated by the liquid flowing through the measurement chamber, it transmits this rotary motion to a totalizer, either by means of a magnetic drive system or else mechanically via a gear train.
Because of their weight and their repulsion or attraction forces, the magnets of a magnetic drive system put a mechanical torque on the meter that opposes any setting into motion of the spinner, particularly at low flow rates. This has the consequence of increasing the error in the accuracy of measurements performed by the meter at low flow rates.
To solve this problem attempts may be made to increase the number of spinner blades or to increase the number of radial ribs disposed on the bottom and/or the top portion of the measurement chamber, or indeed to dispose a plurality of slots in the housing in association with each of the tubes. However, that does not reduce the error in the accuracy of measurements performed by the meter at low flow rates.